The max q condition is the point when an aerospace vehicle's atmospheric flight reaches maximum dynamic pressure. This is a significant factor in the design of such vehicles because the aerodynamic structural load on them is proportional to dynamic pressure. This may impose limits on the vehicle's flight envelope.
Dynamic pressure, q, is defined mathematically as
where ρ is the local air density, and v is the vehicle's velocity; the dynamic pressure can be thought of as the kinetic energy density of the air with respect to the vehicle. For a launch of a rocket from the ground into space, dynamic pressure is
- zero at lift-off, when the air density ρ is high but the vehicle's speed v = 0
- zero outside the atmosphere, where the speed v is high, but the air density ρ = 0
- always non-negative, given the quantities involved
Therefore, (by Rolle's theorem) there will always be a point where the dynamic pressure is maximum.
In other words, before reaching max q, the dynamic pressure change due to increasing velocity is greater than that due to decreasing air density so that the dynamic pressure (opposing kinetic energy) acting on the craft continues to increase. After passing max q, the opposite is true. The dynamic pressure acting against the craft decreases as the air density decreases, ultimately reaching 0 when the air density becomes zero.
Rocket launch examples
During a normal Space Shuttle launch, for example, max q value of 0.32 atmospheres occurred at an altitude of approximately 11 km (36,000 ft). The three Space Shuttle Main Engines were throttled back to about 60–70% of their rated thrust (depending on payload) as the dynamic pressure approached max q; combined with the propellant grain design of the solid rocket boosters, which reduced the thrust at max q by one third after 50 seconds of burn, the total stresses on the vehicle were kept to a safe level.
During a typical Apollo mission, the max q (also just over 0.3 atmospheres) occurred between 13 and 14 kilometres (43,000–46,000 ft) of altitude; approximately the same values occur for the SpaceX Falcon 9.
The point of max q is a key milestone during a rocket launch, as it is the point at which the airframe undergoes maximum mechanical stress.
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- Heiney, Anna (2007-08-08). "Launch Blog". NASA. Retrieved 2011-05-22.
- Woods, David; O'Brien, Frank (2005-08-21). "Apollo 8, Day 1: Launch and Ascent to Earth Orbit". Apollo Flight Journal. NASA. Archived from the original on 2013-07-02. Retrieved 2007-02-14.
- Brandt, Tim; Woods, David (2004-10-29). "Apollo 16, Day One Part One: Launch and Reaching Earth Orbit". Apollo Flight Journal. NASA. Archived from the original on 2013-07-02. Retrieved 2007-02-14.
- "Starlink Mission (the moment Falcon 9 goes through MAX-Q at an altitude of 12.7 km)".